Surface-water samples for chemical analyses were collected during May and August 1995 from 13 sites in an irrigated area of the Wind River Federal Irrigation Project, Wind River Indian Reservation, Wyoming. Sites included irrigation drains, ponds, and streams in an irrigated area partially underlain by seleniferous geologic materials described as having the potential to support vegetation that may be toxic to animals. Samples were analyzed for dissolved selenium, mercury, and other inorganic constituents. The study was done in cooperation with the Shoshone Tribe and the Northern Arapaho Tribe.
Six samples analyzed from the May sampling period had dissolved selenium concentrations greater than the aquatic-life chronic criterion of 5 μg/L (micrograms per liter) established by the U.S. Environmental Protection Agency (USEPA). The largest dissolved selenium concentration (49 μg/L) exceeded the aquatic-life acute criterion of 20 μg/L established by the USEPA. Selenium concentrations in all 12 samples collected during the August sampling period were less than the laboratory minimum reporting level of 5 μg/L. The Goose Pond Drain site was dry in August.
The dissolved solids concentrations were larger in samples collected in May than in August from all but two sites. The large dissolved selenium concentrations in the May samples, collected prior to the start of the irrigation season, were associated with large dissolved solids concentrations.
Dissolved mercury concentrations were less than the minimum reporting level of 0.1 jig/L for 24 of the 25 samples collected during May and August. One sample collected in May had a concentration of 0.3 μg/L. This concentration was less than the aquatic-life acute criterion of 2.4 μg/L for mercury established by the USEPA.
